Composite photoalignment layer

ABSTRACT

A composite photoalignment layer for aligning liquid crystal molecules includes: a monomeric material; a photoinitiator or a thermal initiator; and an azo dye material. A method for preparing a composite photoalignment layer for aligning liquid crystal molecules includes: mixing, in solution form, a monomeric material, a photoinitiator or a thermal initiator, and an azo dye material; coating the mixed solution onto a substrate to form a thin film; exposing the thin film to polarized light; and, with a thermal initiator, heating the thin film to polymerize the monomeric material and form a solid thin film.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation-in-part of InternationalApplication No. PCT/CN2016/103739, filed on Oct. 28, 2016, which claimspriority to U.S. Provisional Patent Application No. 62/285,435, filedOct. 29, 2015, and U.S. Provisional Patent Application No. 62/493,840,filed Jul. 19, 2016. This patent application also claims priority toU.S. Provisional Patent Application No. 62/496,381, filed Oct. 17, 2016.The disclosures of the aforementioned applications are herebyincorporated by reference in their entireties.

BACKGROUND

In-plane switching displays, fringe field switching displays, and fieldsequential color displays based on ferroelectric liquid crystal displayhave recently become more popular because of their ability to providerelatively high optical quality and resolution, and it is desirable tofor display cells to have a fast response time, a wide viewing angle,and high resolution. For example, the use of electrically suppressedhelix ferroelectric liquid crystals provides great optical quality (likenematic liquid crystals), with a relatively fast switching response anda relatively low driving voltage.

Applications of liquid crystal display cells having fast response, highresolution and high optical contrast may include, for example, fastresponse photonics devices such as modulators, filters, attenuators, anddisplays with high resolution requirements (e.g., pico projectors, 3Ddisplays, microdisplays, high-definition televisions (HDTVs), ultrahigh-definition (UHD) displays, etc.).

SUMMARY

In an exemplary embodiment, the invention provides a compositephotoalignment layer for aligning liquid crystal molecules, including: amonomeric material; a photoinitiator; and an azo dye material.

In another exemplary embodiment, the invention provides a method forpreparing a composite photoalignment layer for aligning liquid crystalmolecules, the method including: mixing, in solution form, a monomericmaterial, a photoinitiator, and an azo dye material; coating the mixedsolution onto a substrate to form a thin film; and exposing the thinfilm to polarized light to form a solid thin film.

In yet another exemplary embodiment, the invention provides a compositephotoalignment layer for aligning liquid crystal molecules, including: amonomeric material; a thermal initiator; and an azo dye material.

In yet another exemplary embodiment, the invention provides a method forpreparing a composite photoalignment layer for aligning liquid crystalmolecules, the method including: mixing, in solution form, a monomericmaterial, a thermal initiator, and an azo dye material; coating themixed solution onto a substrate to form a thin film; exposing the thinfilm to polarized light to impose a single-domain or multi-domainalignment; and heating the thin film to polymerize the monomericmaterial and form a solid thin film.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a schematic example of an exemplary process for preparinga composite photoalignment layer for aligning liquid crystal moleculesaccording to a first exemplary embodiment.

FIGS. 2A-2B show transmittance against voltage curves (TVCs) for anexemplary twisted nematic (TN) display cell before and after thermalexposure.

FIGS. 3A-3B show the TVCs for an exemplary electrically-controlledbirefringence (ECB) nematic display cell before and after thermalexposure.

FIGS. 4A-4B show the TVCs for an exemplary TN display cell before andafter photo exposure.

FIGS. 5A-5B show the TVCs for an exemplary ECB nematic display cellbefore and after photo exposure.

FIG. 6 is an image depicting an example of the optical texture of amulti-domain alignment.

FIG. 7 depicts a schematic example of an exemplary process for preparinga composite photoalignment layer for aligning liquid crystal moleculesaccording to a second exemplary embodiment.

FIG. 8 shows the TVCs for an exemplary TN display cell before and afterthermal exposure.

FIG. 9 shows the TVCs for an exemplary ECB nematic display cell beforeand after thermal exposure.

FIG. 10 shows the TVCs for an exemplary TN display cell before and afterphoto exposure.

FIG. 11 shows the TVCs for an exemplary ECB nematic display cell beforeand after photo exposure.

FIG. 12 is a plot showing the time-dependence of residual direct current(RDC) voltage of an exemplary composite photoalignment layer afterstress of 10V for 1 hour.

FIG. 13 is an image depicting an example of the optical texture of amulti-domain alignment.

FIG. 14 is a graph depicting in situ phase retardation for an exemplaryphoto-polymerized composite photoalignment layer at differentenvironmental humidity levels (ranging from 10% to 80% relativehumidity) as a function of exposure time.

FIG. 15 is a graph depicting phase retardation for an exemplaryphoto-polymerized composite photoalignment layer as a function ofrelative humidity after being exposed with linearly polarized 365 nm UVlight with an exposure dosage of 2 J/cm².

FIG. 16 depicts images of various examples of TN LC cells, under crossedpolarizers, the TN LC cells being made using orthogonally-alignedcomposite photoalignment layers (polymerized via photo polymerization)on respective substrates of the TN LC cells corresponding to differentenvironmental humidity levels (20%, 40%, 50%, 55%, 60%, 80%) under thesame exposure conditions (linearly polarized 365 nm UV light with anexposure dosage of 2 J/cm²).

FIG. 17 is a graph depicting in situ phase retardation for an exemplarythermally-polymerized composite photoalignment layer at differentenvironmental humidity levels (ranging from 30% to 70% relativehumidity) as a function of exposure time.

FIG. 18 is a graph depicting phase retardation for an exemplarythermally-polymerized composite photoalignment layer as a function ofrelative humidity after being exposed with linearly polarized 365 nm UVlight with an exposure dosage of 0.1 J/cm².

FIG. 19 depicts images of various examples of TN LC cells, under crossedpolarizers, the TN LC cells being made using orthogonally-alignedcomposite photoalignment layers (polymerized via photo polymerization)on respective substrates of the TN LC cells corresponding to differentenvironmental humidity levels (20%, 40%, 60%, 70%, 80%) under the sameexposure conditions (linearly polarized 365 nm UV light with an exposuredosage of 0.1 J/cm²).

FIG. 20 is a graph depicting in situ phase retardation for an exemplaryphotoalignment layer at different environmental humidity levels (rangingfrom 10% to 80% relative humidity) as a function of exposure time tolinearly polarized 365 nm UV light.

FIG. 21 is a graph depicting phase retardation for an exemplaryphotoalignment layer as a function of relative humidity after beingexposed with linearly polarized 365 nm UV light with an exposure dosageof 1 J/cm² and 3 J/cm².

FIG. 22 is a graph depicting dichroic ratio and order parameter for aphotoalignment layer as a function of relative humidity after beingexposed with linearly polarized 365 nm UV light with an exposure dosageof 1 J/cm².

FIG. 23 depicts images of various examples of TN LC cells, under (a)crossed polarizers and (b) parallel polarizers, the TN LC cells beingmade using orthogonally photoaligned layers corresponding to differentenvironmental humidity levels (40%, 60%, and 80% relative humidity fromleft to right) under the same exposure conditions (linearly polarized365 nm UV light with an exposure dosage of 1 J/cm²).

DETAILED DESCRIPTION

The electro-optical modes and pixel structure manipulations needed forcertain liquid crystal display cells having fast response, highresolution and high optical contrast may demand highly optimizedphotoalignment to provide zero pre-tilt angle, large surface uniformityand multi-domain alignment (multi-domain alignment in a pixel improvesvisual appearance and viewing characteristics, including viewing anglecharacteristics).

Conventional photoalignment materials are not able to offer all of thesequalities. Conventional azo dye alignment layers are able to providegood alignment (with high anchoring energy, small pre-tilt angle, anduniformity over a relatively large area) for liquid crystals in displaycells, allowing the liquid crystal display cells to achieve very highpixel resolution. However, conventional azo dye alignment layers are notstable against chemical, thermal and photo exposure.

Exemplary embodiments of the invention provide a compositephotoalignment layer for liquid crystals, the composite photoalignmentlayer including a composite mixing of at least a monomer (“monomericmaterial”), a thermal free radical initiator (“thermal initiator”) or aphotoinitiator, and an azo dye material (such as an SD1 azo dye). Byintroducing a polymer network into the azo dye material (viathermally-initiated or photoinitiated polymerization), exemplaryembodiments of the invention provide a stabilized composite azo dyephotoalignment layer which is stable against ultraviolet light exposure,heat, and other environmental conditions.

The composite photoalignment layer provides good alignmentcharacteristics (e.g., low pretilt angle, high polar and azimuthalanchoring energy, low residual direct current (RDC) voltage, highvoltage holding ratio (VHR), low image sticking parameter), comparableto that of conventional polyimide layers, and meets industry andconsumer standards (e.g., with respect to RDC voltage, VHR and anchoringenergy). The composite photoalignment layer is adaptable tosophisticated surfaces and is easy to pattern, and is thus suitable foruse in a variety of photonic elements and displays, including but notlimited to in-plane switching (IPS) and ferroelectric liquid crystal(FLC) displays, as well as photonics devices such as gratings,modulators, and polarization converters.

In a first exemplary embodiment, starting with a mixture of a monomer, aphotoinitiator, and an azo dye material (at concentrations configured toprovide stability for the azo dye material without affecting thealignment provided by the photoalignment layer), and by using a singlelight exposure to provide both photoinduced reorientation of the azo dyematerial (photoalignment) and polymerization of the monomer, a compositephotoalignment layer with good alignment characteristics (e.g., highanchoring energy, small pre-tilt angle, and uniformity over a relativelylarge area) is achieved. The composite photoalignment layer is thusformed in a single step irradiation/exposure, and provides a good andstable photoalignment for liquid crystals.

In a second exemplary embodiment, the process starts with a mixture of amonomer, a thermal initiator, and an azo dye material (at concentrationsconfigured to provide stability for the azo dye material withoutaffecting the alignment provided by the photoalignment layer). Then, ina first step, a preferred orientation of the easy axis of the azo dyephotoalignment layer is realized. In a second step, thermalpolymerization is performed.

Photoalignment provides the ability to realize single-domain ormulti-domain alignment with an extremely small pretilt angle in a singlestep of irradiation/exposure. Using a single-step photoalignment processwith, for example, a patterned wave plate, a multi-domain photoalignmentlayer may be achieved with highly uniform alignment over a large size.Further, because the azo dye material offers only in-plane moleculardiffusion from one direction to another, and does not go out of plane,the generated pre-tilt angle is very small.

Additionally, according to exemplary embodiments of the invention, theanchoring energies of the composite photoalignment layer are adjustableby controlling the irradiation dosage, which allows for optimization ofalignment quality, for example, for nematic LCs and ferroelectric LCs.Thus, exemplary embodiments of the invention are suitable forapplications requiring precise control of anchoring energies, includingbut not limited to, for example, ferroelectric liquid crystal displays.

A liquid crystal photoalignment layer shows a preferred alignmentdirection after being irradiated by polarized light with sufficientlyhigh irradiation energy of certain wavelength (the polarized lightimposes an alignment direction on the photoalignment layer).Photoalignment provides several advantages over conventional rubbingalignment techniques. For example, rubbing may cause mechanical damageor electrostatic charge, which degrades manufacturing yield.Photoalignment avoids mechanical contact with the aligning layer, andthus minimizes such mechanical damage and electrostatic charging.Photoalignment is also easier to implement with respect to largesubstrates and provides better uniformity for high resolution displays.Additionally, photoalignment provides the ability to realizemulti-domain alignment on a micro-scale or even on a nano-scale.Furthermore, photoalignment may be utilized with respect to a non-flatsurface such as a curved or flexible surface (e.g., for curved LCDpanels or flexible displays) or surfaces with microscopic confinements.

There are several approaches to photoalignment, including for example,the following categories: (1) photoalignment by cis-trans isomerizationof azo dye molecules; (2) photocrosslinking of monomers into polymers;(3) photo-degradation of a polymer layer; and (4) photoinducedreorientation of azo dye molecules. Among these, photoinducedreorientation of azo dye molecules provides certain advantages forexample, sufficiently high polar and azimuthal anchoring energies forliquid crystal alignment, which may be as strong as a commercialpolyimide film based on conventional rubbing; high voltage holding ratio(VHR) and low residual direct current (RDC) voltage is low, which isadvantageous for liquid crystal alignment; and very small pretilt angle(e.g., less than 1 degree), which is advantageous for display modes thatrequire such low pretilt angles, such as the in-plane switching (IPS)mode and derivatives thereof such as the fringe-field switching (FFS)mode. Further, photoinduced reorientation of azo dyes may be achievedwith polarized light over a large range of wavelengths, including forexample blue light or ultraviolet light. This allows high powerlight-emitting diodes (LEDs) to be used as the light source so as toreduce the cost of the photoalignment equipment. Further, photoinducedreorientation of azo dyes is applicable to optically rewritabledisplays, where the rewritable property of the azo dye allows forwriting and erasing images as desired.

Photoalignment based on photoinduced reorientation of azo dye moleculesis thus able to achieve sufficiently high polar and azimuthal anchoringenergy, high VHR, appropriate pre-tilt angles, and uniform alignment.Additionally, photoalignment based on photoinduced reorientation of azodye molecules is easily rotatable using blue light and providesanchoring energy comparable to a commercial polyimide film with very lowpretilt angle. Photoalignment based on photoinduced reorientation of azodye molecules may be used in a wide range of LC devices, including forexample, IPS and FLC displays. Photoalignment based on photoinducedreorientation of azo dye molecules is tunable based on controlling theirradiation dosage. Photoalignment based on photoinduced reorientationof azo dye molecules is further able to provide a multi-domain alignmentwith a distinctly defined easy axis of the alignment. Additionally,photoalignment based on photoinduced reorientation of azo dye moleculesprovides the ability to align nanoscopic domains so as to provide forbetter viewing, optical and other characteristics of liquid crystaldisplays.

However, as mentioned above, the photo-degradation and instability ofconventional azo dye photoalignment layers hinders the deployment of azodye photoalignment layers in certain real world applications. Inparticular, if a photoaligned display cell is exposed to light, the easyaxis of the azo dye photoalignment layer may change and damage thealignment quality of the display cell. Further, light flux from thebacklight of a display system may be strong enough to damage thealignment characteristics of the photoalignment layer within a few hoursof operation.

In the first exemplary embodiment, the invention provides a compositephotoalignment layer for liquid crystals that comprises a monomer, aphotoinitiator, and an azo dye material in optimal relativeconcentrations. The composite photoalignment layer provides good,uniform alignment and is stable after being irradiated by a lightsource. The concentration of the photoinitiator and the monomer aretuned to provide both alignment and stabilization in a singleirradiation.

In an exemplary implementation, the monomer has liquid crystalproperties and is a liquid crystalline reactive mesogen; the azo dye issulfonic dyetetrasodium5,5′-((1E,1′E)-(2,2′-disulfonato-[1,1′-biphenyl]-4,4′-diyl)bis(diazene-2,1-diyl))bis(2-hydroxybenzoate)(“SD1”); and the photoinitiator is 1-hydroxycyclohexyl phenyl ketone. Itwill be appreciated that in other exemplary implementations, othermaterials may be used. For example, in another exemplary embodiment, thephotoinitiator may be 2,2-dimethoxy-1,2-diphenyl ethanone.

In one example, the process of making the composite photoalignment layerbegins with mixing the monomer and azo dye at optimal relativeconcentrations of 50:50 (since the molecule length of the azo dye andthe monomer is approximately the same). Then, the photoinitiator at 10%wt/wt of the monomer is added to the mixture. It will be appreciatedthat in other exemplary implementations and that with other materials,other relative concentrations of materials may be used.

The concentration of photoinitiator is tuned to optimize the rate ofpolymerization (e.g., to ensure that polymerization is not completedbefore photoalignment, which would negatively affect the opticalquality). In various exemplary implementations, the concentration ofphotoinitiator that is added to the mixture may be varied between 1%wt/wt of the monomer to 30% wt/wt of the monomer in the solvent tooptimize the balance between the rate of alignment (to achieve a certainamount of liquid crystal anchoring energy) and the rate ofpolymerization. Further, based on the relationship between theabsorption band of the photoinitiator and the absorption band of the azodye, different balances between the rate of alignment and the rate ofpolymerization may be achieved. In one example, the photoinitiatorabsorption band is chosen to match the absorption band of the azo dye(e.g., SD1 azo dye has absorption peaks at 365 nm and 450 nm). In otherexamples, the absorption band of the photoinitiator is different fromthe absorption band of the azo dye.

Additionally, the azimuthal anchoring energy of the compositephotoalignment layer can be tuned by varying the irradiation energy aswell as by balancing the rate of the alignment and the rate ofpolymerization.

A process for preparing a composite photoalignment layer for aligningliquid crystal molecules includes: mixing, in solution form, a monomericmaterial, a photoinitiator, and an azo dye material; coating the mixedsolution onto a substrate to form a thin film; and exposing the thinfilm to polarized light to form a solid thin film. Exposing the thinfilm is a single step exposure that provides both alignment andpolymerization for the composite photoalignment layer. Thephotoalignment layer may be coated onto a substrate surface based on avariety of coating techniques, including but not limited to, forexample, spin coating, doctor blading, and screen printing. Thepolarized light may be from a polarized light source having one or moremajor wavelength components (e.g., such that separate irradiation bandsfor alignment and polymerization may be used).

FIG. 1 depicts a schematic example of this process. As shown in FIG. 1,a mixture of SD1 azo dye, monomer and photoinitiator, composited in asolvent (e.g., dimethylformamide (DMF)), in solution form, is spincoated onto a substrate at stage 101 so as to form a thin film at stage102. Then, at stage 103, the thin film is exposed in a single stepexposure that provides both alignment and polymerization for thecomposite photoalignment layer so as to form a solid thin film havingthe SD1 molecules and a polymer network formed from the monomers atstage 104. In particular, the polymerization of the monomeric materialin the composite photoalignment layer causes the compositephotoalignment layer to form a solid thin film, and polymerization ofthe monomeric material provides high liquid crystal anchoring energy(e.g., ˜10⁻³ J/m²). It will be appreciated that the monomeric materialmay be fully polymerized in accordance with exemplary embodiments of theinvention.

The particular level of the anchoring energy may be tuned based on theirradiation dosage. In one example, an anchoring energy in the range of10⁻⁵ J/m² to 10⁻² J/m² may be achieved (e.g., approximately on the orderof magnitude of 10⁻⁵ J/m² or 10⁻³ J/m²). Further, it will be appreciatedthat the anchoring energy may be tuned within the range of 10⁻⁵ J/m² to10⁻² J/m² by adjusting the irradiation dosage.

In an exemplary implementation, the composite photoalignment layermanifests low RDC voltage, e.g., under 10 mV.

In an exemplary implementation, the composite photoalignment layerprovides electro-optical characteristics that are the same or similar toconventional polyimide alignment layers. In an example, the voltageholding ratio for a planar aligned nematic liquid crystal cell havingthe composite photoalignment layer is greater than 99% for a frame rateof 60 Hz.

In an exemplary implementation, the composite photoalignment layerprovides alignment quality that is comparable to conventional andcommercially available alignment layers.

In an exemplary implementation, the composite photoalignment layer, withfull polymerization of the monomer, provides an image sticking parameter(“ISP”) ratio of 1.01, which is comparable to conventional alignmentlayers. The image sticking parameter defines how a display panel behavesagainst a ghost image of a previous frame. In an example, it wasdemonstrated that the ISP ratio is 1.01 based on application of a stressof 6V being applied to one of two pixels of a cell for 6 hours, with theother pixel being left at 0V, and comparing the transmittance of the twopixels at a stress of 2V.

In an exemplary implementation, the composite photoalignment layer wasdemonstrated as being thermally stable in that it did not reveal anytraces of degradation after thermal exposure at 100° C. for 24 hours inan oven. As shown in FIGS. 2A-2B and FIGS. 3A-3B, the transmittanceagainst voltage curves (TVCs) for exemplary display cells having thecomposite photoalignment layer were unaffected after the thermalexposure. FIGS. 2A-2B show the TVCs for an exemplary twisted nematic(TN) display cell before and after thermal exposure. FIGS. 3A-3B showthe TVCs for an exemplary electrically-controlled birefringence (ECB)nematic display cell before and after thermal exposure. The alignmentquality of the exemplary display cells were also unaffected by thethermal exposure, as was apparent from visual inspection.

The composite photoalignment layer was also demonstrated as beingoptically stable and did not show any degradation after photo exposureto a light source with intensity 100 mW/cm² for 1 hour. As shown inFIGS. 4A-4B and FIGS. 5A-5B, the TVCs for exemplary display cells havingthe composite photoalignment layer were unaffected after the photoexposure. FIGS. 4A-4B show the TVCs for an exemplary TN display cellbefore and after photo exposure. FIGS. 5A-5B show the TVCs for anexemplary ECB nematic display cell before and after photo exposure. Thealignment quality of the exemplary display cells were also unaffected bythe photo exposure, as was apparent from visual inspection.

In an exemplary implementation, during the single step exposure at stage103 of FIG. 1, a phase mask is used to provide two or more alignmentdomains for the composite photoalignment layer. In an example, apatterned half wave plate with two domains with characteristic size of20 μm is used to provide the phase mask. The phase mask rotates theplane of the impinging light and thereafter the impinging light, withdegenerated plane of polarization, exposes the substrate coated with thecomposite photoalignment layer. As a result, the irradiated substrateprovides multi-domain alignment that is stable and resistant to thermaland photo exposure, while having high quality optical and electricalparameters. An example of the optical texture of a multi-domainalignment is depicted in FIG. 6.

In the second exemplary embodiment, the invention provides a compositephotoalignment layer for liquid crystals that comprises a monomer, athermal initiator, and an azo dye material in optimal relativeconcentrations. The composite photoalignment layer provides good,uniform alignment after being irradiated by a light source and is stableafter being heated (e.g., at 230° C. for 30 minutes, but it will beappreciated that other times and temperatures can be used). Theconcentration of the thermal initiator and the monomer are tuned toprovide both a good alignment and stabilization for the alignment.

In an exemplary implementation, the monomer has liquid crystalproperties and is4-(3-acryloyloxypropyloxy)-benzoesure-2-methyl-1,4-phenylester; the azodye is sulfonic azo dyetetrasodium5,5′-((1E,1′E)-(2,2′-disulfonato-[1,1′-biphenyl]-4,4′-diyl)bis(diazene-2,1-diyl))bis(2-hydroxybenzoate)(“SD1”); and the thermal initiator is 2-cyano-2-propyl dodecyltrithiocarbonate. It will be appreciated that in other exemplaryimplementations, other materials may be used.

In one example, the process of making the composite photoalignment layerbegins with mixing the monomer and azo dye at optimal relativeconcentrations of 50:50 (since the molecule length of the azo dye andthe monomer is approximately the same). Then, the thermal initiator at5% wt/wt of the monomer is added to the mixture. The mixture is furtherdissolved in a solvent (e.g., dimethylformamide or other polarsolvents). It will be appreciated that in other exemplaryimplementations and that with other materials, other relativeconcentrations of materials may be used.

In an exemplary implementation, the concentration of the azo dye andmonomer combined is 1% wt/wt of the solvent, whereas the concentrationof the thermal initiator is 5% wt/wt of the monomer. It will beappreciated that in other exemplary implementations and that with othermaterials, other relative concentrations of materials may be used.

A process for preparing a composite photoalignment layer for aligningliquid crystal molecules includes: mixing, in solution form, a monomericmaterial, a thermal initiator, and an azo dye material; coating themixed solution onto a substrate to form a thin film; exposing the thinfilm with polarized light to impose a single-domain or multiple-domainalignment; and heating the thin film to form a solid thin film. Exposingand heating the thin film may be performed simultaneously as part of asingle step or sequentially in separate steps. The thermalpolymerization caused by heating the thin film does not affect thealignment properties (such as anchoring energy and surface uniformity)of the composite photoalignment layer.

FIG. 7 depicts a schematic example of this process. As shown in FIG. 7,a mixture of SD1 azo dye, monomer and thermal initiator, in solutionform, is spin coated onto a substrate at stage 701 so as to form a thinfilm at stage 702. Then, at stage 703, the thin film is exposed in asingle step exposure that provides alignment for the compositephotoalignment, and at stage 704, the thin film is heated at 230° C. for30 minutes, so as to form a solid thin film having the SD1 molecules anda polymer network formed from the monomers at stage 705. In particular,the polymerization of the monomeric material in the compositephotoalignment layer causes the composite photoalignment layer to form asolid thin film, and polymerization of the monomeric material provideshigh liquid crystal anchoring energy (e.g., ˜10⁻³ J/m²). It will beappreciated that the monomeric material may be fully polymerized inaccordance with exemplary embodiments of the invention.

The particular level of the anchoring energy may be tuned based on theirradiation dosage. For example, an anchoring energy in the range of10⁻⁵ J/m² to 10⁻² J/m² may be achieved (e.g., approximately on the orderof magnitude of 10⁻⁵ J/m² or 10⁻³ J/m²). In another example, ananchoring energy of approximately 3×10⁻³ J/m² may be achieved. Further,it will be appreciated that the anchoring energy may be tuned within therange of 10⁻⁵ J/m² to 10⁻² J/m² by adjusting the irradiation dosage.

In an exemplary implementation, the composite photoalignment layerprovides electro-optical characteristics that are the same or similar toconventional polyimide alignment layers. In an example, the voltageholding ratio for an electrical controlled birefringence liquid crystalcell having the composite photoalignment layer is greater than 99% for aframe rate of 60 Hz.

In an exemplary implementation, the composite photoalignment layerprovides alignment quality that is comparable to conventional andcommercially available alignment layers.

In an exemplary implementation, the composite photoalignment layer wasdemonstrated as being thermally stable in that it did not reveal anytraces of degradation after thermal exposure at 100° C. for 24 hours inan oven. As shown in FIGS. 8 and 9, the TVCs for exemplary display cellshaving the composite photoalignment layer were unaffected after thethermal exposure. FIG. 8 shows the TVCs for an exemplary TN display cellbefore and after thermal exposure. FIG. 9 shows the TVCs for anexemplary ECB nematic display cell before and after thermal exposure.The alignment quality of the exemplary display cells were alsounaffected by the thermal exposure, as was apparent from visualinspection.

The composite photoalignment layer was also demonstrated as beingoptically stable and did not show any degradation after photo exposureto a light source with 400 J/cm² of energy at a wavelength of 450 nm. Asshown in FIGS. 10 and 11, the TVCs for exemplary display cells havingthe composite photoalignment layer were unaffected after the photoexposure. FIG. 10 shows the TVCs for an exemplary TN display cell beforeand after the photo exposure. FIG. 11 shows the TVCs for an exemplaryECB nematic display cell before and after the photo exposure. Thealignment quality of the exemplary display cells were also unaffected bythe photo exposure, as was apparent from visual inspection.

In an exemplary implementation, the composite photoalignment layermanifests low RDC voltage, e.g., under 10 mV in an example where a DCsoak of 10V is performed for an hour at 60° C. FIG. 12 shows thetime-dependence of the RDC voltage of an exemplary compositephotoalignment layer after stress of 10V for 1 hour.

In an exemplary implementation, during the single step exposure at stage703 of FIG. 7, a phase mask is used to provide two or more alignmentdomains with distinct alignment directions in neighboring domains forthe composite photoalignment layer. As a result, the irradiatedsubstrate provides multi-domain alignment that is stable and resistantto thermal and photo exposure, while having high quality optical andelectrical parameters. An example of the optical texture of amulti-domain alignment having a checker board pattern with acharacteristic size of 20 μm is depicted in FIG. 13.

Exemplary embodiments of the invention thus provide a compositephotoalignment layer with full polymerization of the monomer, whileproviding acceptable values for residual DC voltage, image stickingparameter, and voltage holding ratio. In an example, a compositephotoalignment layer with full polymerization of the monomer provides aminimum and acceptable residual DC voltage value of 0.008 V, a minimumand acceptable image sticking parameter ratio of 1.01, and a minimum andacceptable voltage holding ratio of more than 99% at 60° C. and 60 Hzframe frequency.

In certain exemplary embodiments, alignment quality is correlated withthe environmental humidity level at which the photoalignment materials(e.g., azo dye material, photo-polymerized azo dye composite layers, andthermally-polymerized azo dye composite layers) are processed. Forexample, LC cells for which processing was performed at differentenvironmental humidity levels may exhibit different behaviors—e.g.,relating to photo-induced phase retardation, an order parameter of thealignment layer, and/or alignment quality.

To achieve a desired environmental humidity level for testing theeffects of environmental humidity level during processing aphotoalignment material, in an exemplary embodiment, the photoalignmentmaterial is prepared in an Argon-filled glove box, wherein the watermolecules in the environment are controlled to within 0.5 ppm (withrelative humidity being lower than 1%). The photoalignment material isdissolved in solvent and spin-coated onto a substrate to form a uniformthin film within the glove box. The film is then put into a sealedchamber and taken out of the glove box. The sealed chamber is thenconnected with a humidity generator to achieve equilibrium at a desiredenvironmental humidity. Then, the film is exposed to polarized UV lightto create a preferred alignment direction.

In an exemplary embodiment, for a composite photoalignment layer whichis polymerized using a photo-initiator (for example, as discussed abovein connection with FIG. 1), a desirable environmental relative humidityprocessing window may be, for example, 50%-75%, with 55%-70% relativehumidity being preferred to achieve good planar alignment quality. FIG.14 is a graph depicting in situ phase retardation for aphoto-polymerized composite photoalignment layer at differentenvironmental humidity levels (ranging from 10% to 80% relativehumidity) as a function of exposure time, showing the dynamic change ofphase retardation as a function of time (exposure energy), where theexposure energy increases from 0 to about 9 J/cm². FIG. 15 is a graphdepicting phase retardation for a photo-polymerized compositephotoalignment layer as a function of relative humidity after beingexposed with linearly polarized 365 nm UV light with an exposure dosageof 2 J/cm². After exposure, the photoaligned films were soft baked at120° C. for 10 min to evaporate the solvent. FIG. 16 depicts images ofvarious examples of TN LC cells, under crossed polarizers, the TN LCcells being made using orthogonally-aligned composite photoalignmentlayers (polymerized via photo polymerization) on respective substratesof the TN LC cells corresponding to different environmental humiditylevels (20%, 40%, 50%, 55%, 60%, 80%) under the same exposure conditions(linearly polarized 365 nm UV light with an exposure dosage of 2 J/cm²).In some situations at low humidity levels, homeotropic alignment, ratherthan planar alignment, was observed.

In an exemplary embodiment, for a composite photoalignment layer whichis polymerized using a thermal initiator (for example, as discussedabove in connection with FIG. 1), a desirable environmental humiditywindow to achieve good planar alignment quality may be, for example,0-75%, with 0-70% relative humidity being preferred to achieve goodplanar alignment quality. In an exemplary implementation, linearlypolarized UV exposure (at a wavelength of 365 nm) of 0.1 J/cm² is usedfor photo-reorientation, and the composite photoalignment layer is thenhard baked for thermal polymerization at 230° C. for 30 minutes. FIG. 17is a graph depicting in situ phase retardation for athermally-polymerized composite photoalignment layer at differentenvironmental humidity levels (ranging from 30% to 70% relativehumidity) as a function of exposure time, showing the dynamic change ofphase retardation as a function of time (exposure energy), where theexposure energy increases from 0 to about 9 J/cm². FIG. 18 is a graphdepicting phase retardation for a thermally-polymerized compositephotoalignment layer as a function of relative humidity after beingexposed with linearly polarized 365 nm UV light with an exposure dosageof 0.1 J/cm². FIG. 19 depicts images of various examples of TN LC cells,under crossed polarizers, the TN LC cells being made usingorthogonally-aligned composite photoalignment layers (polymerized viaphoto polymerization) on respective substrates of the TN LC cellscorresponding to different environmental humidity levels (20%, 40%, 60%,70%, 80%) under the same exposure conditions (linearly polarized 365 nmUV light with an exposure dosage of 0.1 J/cm²).

It will be appreciated that the environmental humidity range forprocessing photoalignment materials discussed herein is also applicablein other contexts, for example, with respect to other types of photonicdevices and displays. In an exemplary embodiment, for a photoalignmentlayer which is just an azo dye film (e.g., an SD1 film), a desirableenvironmental humidity window to achieve good planar alignment qualitymay be, for example, 40%-75% relative humidity, with 50%-70% relativehumidity being preferred to achieve good planar alignment quality. Toachieve a desired environmental humidity level for testing the effectsof environmental humidity level during processing a photoalignmentmaterial, in an exemplary implementation, a photoalignment film isspin-coated onto a substrate and then soft baked at 100° C. for 10 minin a glove box. The photoalignment film is then put in a sealed chamberand taken out of the glove box. FIG. 20 is a graph depicting in situphase retardation for a photoalignment layer at different environmentalhumidity levels (ranging from 10% to 80% relative humidity) as afunction of exposure time to linearly polarized 365 nm UV light, showingthe dynamic change of phase retardation as a function of time (exposureenergy), where the exposure energy increases from 0 to about 9 J/cm².FIG. 21 is a graph depicting phase retardation for a photoalignmentlayer as a function of relative humidity after being exposed withlinearly polarized 365 nm UV light with an exposure dosage of 1 J/cm²and 3 J/cm². FIG. 22 is a graph depicting dichroic ratio and orderparameter for a photoalignment layer as a function of relative humidityafter being exposed with linearly polarized 365 nm UV light with anexposure dosage of 1 J/cm². FIG. 23 depicts images of various examplesof TN LC cells, under (a) crossed polarizers and (b) parallelpolarizers, the TN LC cells being made using orthogonally photoalignedlayers corresponding to different environmental humidity levels (40%,60%, and 80% relative humidity from left to right) under the sameexposure conditions (linearly polarized 365 nm UV light with an exposuredosage of 1 J/cm²). The alignment quality of each of the TN LC cells canbe seen in FIG. 23, with 60% relative humidity corresponding to the bestalignment quality (with high ordering and strong anchoring to the liquidcrystals) in this example (between crossed polarizers, TN LC cellshaving good alignment quality appear bright, and between parallelpolarizers, TN LC cells appear dark).

All references, including publications, patent applications, andpatents, cited herein are hereby incorporated by reference to the sameextent as if each reference were individually and specifically indicatedto be incorporated by reference and were set forth in its entiretyherein.

The use of the terms “a” and “an” and “the” and “at least one” andsimilar referents in the context of describing the invention (especiallyin the context of the following claims) are to be construed to coverboth the singular and the plural, unless otherwise indicated herein orclearly contradicted by context. The use of the term “at least one”followed by a list of one or more items (for example, “at least one of Aand B”) is to be construed to mean one item selected from the listeditems (A or B) or any combination of two or more of the listed items (Aand B), unless otherwise indicated herein or clearly contradicted bycontext. The terms “comprising,” “having,” “including,” and “containing”are to be construed as open-ended terms (i.e., meaning “including, butnot limited to,”) unless otherwise noted. Recitation of ranges of valuesherein are merely intended to serve as a shorthand method of referringindividually to each separate value falling within the range, unlessotherwise indicated herein, and each separate value is incorporated intothe specification as if it were individually recited herein. All methodsdescribed herein can be performed in any suitable order unless otherwiseindicated herein or otherwise clearly contradicted by context. The useof any and all examples, or exemplary language (e.g., “such as”)provided herein, is intended merely to better illuminate the inventionand does not pose a limitation on the scope of the invention unlessotherwise claimed. No language in the specification should be construedas indicating any non-claimed element as essential to the practice ofthe invention.

Preferred embodiments of this invention are described herein, includingthe best mode known to the inventors for carrying out the invention.Variations of those preferred embodiments may become apparent to thoseof ordinary skill in the art upon reading the foregoing description. Theinventors expect skilled artisans to employ such variations asappropriate, and the inventors intend for the invention to be practicedotherwise than as specifically described herein. Accordingly, thisinvention includes all modifications and equivalents of the subjectmatter recited in the claims appended hereto as permitted by applicablelaw. Moreover, any combination of the above-described elements in allpossible variations thereof is encompassed by the invention unlessotherwise indicated herein or otherwise clearly contradicted by context.

The invention claimed is:
 1. A method for preparing a compositephotoalignment layer for aligning liquid crystal molecules, comprising:mixing, in solution form, a monomeric material, a photoinitiator, and anazo dye material, wherein the monomeric material comprises liquidcrystal monomers; coating the mixed solution onto a substrate to form athin film; and exposing the thin film to polarized light to form a solidthin film; wherein exposing the thin film is a single step exposure thatprovides both alignment and polymerization for the compositephotoalignment layer.
 2. The method according to claim 1, wherein thepolarized light is from a polarized light source having one or moremajor wavelength components.
 3. The method according to claim 1, whereinexposing the thin film is performed under environmental humidityconditions in the range of 50%-75% relative humidity.
 4. The methodaccording to claim 3, wherein exposing the thin film is performed underenvironmental humidity conditions in the range of 55%-70% relativehumidity.
 5. The method according to claim 1, wherein the liquid crystalmonomers are liquid crystalline reactive mesogens.
 6. The methodaccording to claim 1, wherein mixing, in solution form, the monomericmaterial, the photoinitiator, and the azo dye material comprises: mixingthe monomeric material and the azo dye material; and adding thephotoinitiator to the mixture of the monomeric material and the azo dyematerial.
 7. The method according to claim 6, wherein the monomericmaterial and the azo dye material are mixed at relative concentrationsof 50:50.
 8. The method according to claim 7, wherein the photoinitiatoris added to the mixture of the monomeric material and the azo dyematerial at a concentration of 10% wt/wt of the monomeric material. 9.The method according to claim 7, wherein the photoinitiator is added tothe mixture of the monomeric material and the azo dye material at aconcentration within the range of 1% wt/wt of the monomeric material to30% wt/wt of the monomeric material.
 10. The method according to claim1, wherein the photoinitiator and the azo dye material have matchingabsorption bands.
 11. The method according to claim 1, wherein a phasemask is used during exposure of the thin film to the polarized light torotate a plane of impinging light.
 12. The method according to claim 11,wherein the phase mask provides two or more alignment domains for thecomposite photoalignment layer.